1. Field of the Invention
The present invention relates to measurement of ionospheric propagation effects on Radio Frequency (RF) signals by comparing carrier phase between signals of different frequency, such as, for example, upper and lower sidebands of a GPS M-code signal.
2. Description of the Related Art
In the upper regions of the earth's atmosphere, ultraviolet and X-ray radiation coming from the sun interact with the atmospheric gas molecules and atoms. These interactions result in ionization giving rise to large numbers of free “negatively charged” electrons and “positively charged” atoms and molecules. The region of the atmosphere where gas ionization takes place is called the ionosphere. It extends from an altitude of approximately 50 km to about 1,000 km or higher (the upper limit of the ionospheric region is not clearly defined).
The electron density within the ionosphere is not constant. It changes with time and altitude. The ionospheric region is typically divided into sub-regions, or layers, according to the electron density. These layers are named D (50-90 km), B (90-140 km), F1 (140-210 km), and F2 (210-1,000 km), respectively, with F2 usually being the layer of maximum electron density. The altitude and thickness of these layers vary with time, as a result of the changes in the sun's radiation and the earth's magnetic field. For example, the F1 layer largely disappears during the night and is more pronounced in the summer than in the winter.
The ionosphere is a dispersive medium, which means that RF waves with the same origination point, but different frequencies, will travel at different speeds and along different ray paths as they pass through the various ionospheric layers. In the case of satellite navigation systems, such as, for example, the Global Positioning System (GPS), bending of the signal propagation path causes a relatively small range error, particularly if the satellite elevation angle is greater than 50 degrees. However, the change in the propagation speed causes a significant range error, and therefore should be accounted for. The ionosphere speeds up the phase velocity of the RF wave. The ionosphere also slows down the group velocity. The code frequency is the fundamental parameter used to determine the space vehicle (SV) range from the receiver while the carrier frequency is primarily used to maintain tracking of the SV signal and to help determine vehicle movement.
The ionospheric delay is proportional to the number of free electrons, called the Total Electron Content (TEC), along the signal path. TEC, however, depends on a number of factors, such as: the time of day; the time of year; the 11-year solar cycle; and the geographic location (electron density levels are minimum in mid-latitude regions and highly irregular in polar, auroral, and equatorial regions). As the ionosphere is a dispersive medium, it causes a delay that is frequency dependent. The delay is greater for lower frequencies than for higher frequencies. Thus, for GPS signals, the ionospheric delay is greater at the L2 carrier frequency than that of the L1 carrier frequency. Generally, ionospheric delay is of the order of 0.5 meters to 15 meters, but can reach over 150 meters under extreme solar activities, at midday, and near the horizon.
Taking advantage of the ionosphere's dispersive nature, the ionospheric delay can be determined with a relatively high degree of accuracy by measuring the “time of flight” between two RF signals of different frequencies that travel along similar paths. In GPS this dual frequency measurements may be accomplished by comparing the P(Y)-code pseudorange measurements between the L1 and L2 frequency bands.
Single frequency band receivers cannot take advantage of the dispersive nature of the ionosphere. They can, however, use an empirical ionospheric model to correct some portion of the error introduced by dispersion. The most widely used model is the Klobuchar model, whose coefficients are transmitted as part of the navigation message. Another solution for users with single-frequency GPS receivers is to use corrections from regional networks. Such corrections can be received in real time through other communication links.